1. Technical Field
This invention relates to the making and use of hydrodynamic or hydrostatic fluid bearings for supporting rotary machining spindles located in such bearings and, more particularly, to the art of hybridizing such hydrodynamic and hydrostatic bearings to permit such bearings to operate effectively at slow speeds as well as variable high speeds up to 40,000 rpm.
2. Discussion of the Prior Art
Hydrostatic liquid or gas bearings alone are useful in providing a fluid film that supports a rotating spindle at relatively low speeds for machining operations. Such bearings may have thick hydrostatic sills extending extensively along a substantial length of the shaft to be supported (see U.S. Pat. No. 5,010,794), or the bearings may have a concentrated, thin, radial port acting as the hydrostatic chamber. Such hydrostatic bearings are limited because they cannot be used under heavy industrial loading at varying rotational spindle speeds where the range between the minimum and maximum intended use speeds for the spindle is greater than 10,000 rpm. Any combination of pressure and viscosity designed into such bearings cannot accommodate both extreme ends of the spindle speed range and thus causes excessive heat to be generated at the extreme high end or failure to form a good bearing film at the other end.
Hydrodynamic fluid bearings alone have found use in supporting machining spindles. Ramped pressure generating zones have been defined on the outer surface of spindles, such ramps extending along a sector of the circumference of the spindle, such as 60.degree., accompanied by a varying profile shape that is extremely difficult to machine (see U.S. Pat. No. 4,693,642). Hydrodynamic bearings inherently are unstable at low spindle speeds due to insufficient dynamic pressure if the spindle loading is heavy; at high spindle speeds, i.e., 20,000-40,000 rpm, the bearing fluid may become thin due to heat and affect bearing performance.
U.S. Pat. No. 4,490,054 has attempted to aggregate hydrostatic and hydrodynamic concepts, each bearing being independent from the other, but aggregation fails to achieve benefits of an integrated hybrid system. Shoes, separated by low pressure chambers and conforming identically to the curvature of the spindle, tilt to create wedging pressure generating zones in one direction; a relieved area in the middle of the shoe allows hydrostatic fluid pressure to flow out of the shoes independently of the hydrodynamic effect. As a result, hydrodynamic pressure is limited.
What is needed is an integrated fluid bearing system that can attain at least 1000 psi hydrodynamically at higher speeds (i.e., 10,000-40,000 rpm) while retaining controlled hydrostatic pressures at lower speeds, thereby providing enhanced stiffness for a spindle subject to large side or thrust loads even at a variety of speeds.